1. Field of the Invention
The present invention relates to an improved compound bow.
2. State of the Prior Art
Compound bows have a number of advantages over conventional archery bows. Because of these advantages, the use of compound bows has increased and compound bows have been gaining in popularity.
In a conventional bow, the force required to draw the bow is determined by the bow's stiffness and the draw force increases in a relatively uniform manner as the bow is drawn. The fact that the force required to draw the bow increases uniformly imposes limitations on the use of the bow. By way of example, if the bow requires a draw force of 50 pounds, the archer must be able not only to draw the bow, but also to hold the bow in a steady position during sighting and discharge of an arrow. While the archer may be able to draw a relatively heavy bow, he may not be able to hold the bow in a steady position during the sighting and release of the arrow while maintaining the force on the bow necessary to keep it in its fully drawn position.
In a compound bow, the draw force does not increase in a uniform manner as the bow is drawn. Through the use of eccentrically mounted pulleys positioned at the ends of the bow limbs, the effective length of the bow limbs is increased during draw of the bow through the rotation of the eccentrically mounted pulleys. By the rotation of the eccentrically mounted pulleys, the force required to maintain the bow in a fully drawn position is decreased with the result that the force required to maintain the bow in the fully drawn position during sighting and release of the arrow is less than the maximum force required in drawing the bow. In the case of a bow having a draw weight, for example, of 50 pounds, the maximum force required in drawing the bow is 50 pounds. However, near the end of the draw, the action of the draw pulleys reduces the force required to hold the bow in the fully drawn position. This "let-off" can range up to 65% or more depending upon the size and eccentricity of the draw pulleys. As the arrow is discharged, the draw pulleys undergo rotational movement which is opposite to the rotational movement that occurs during draw. With the reverse rotational movement of the draw pulleys, the force that is applied to the arrow by the bow is increased, with the result that the bow discharges the arrow under a propulsive force that is higher than the force required to hold the bow during sighting and discharge of the arrow.
The energy which is stored in an archery bow during draw may be determined by integrating the area under the force draw curve of the bow. In the case of a standard bow, the draw force curve increases approximately linearly when the draw force is plotted vertically and the draw distance is plotted horizontally. The force increases continuously as the bow is drawn, with the draw force increasing to a maximum when the bow is fully drawn. However, in a compound bow the draw force curve does not increase linearly, but rather is a curve in which the draw force increases rapidly until a maximum is reached with the draw force then decreasing due to the let-off resulting from the rotation of the eccentrically mounted pulleys. The stored energy, which is represented by the area under the draw force curve, may be greater for a compound bow than the amount of stored energy for an equivalent standard bow having the same draw length and maximum draw force. The compound bow may, therefore, be more efficient in storing energy during the draw of the bow so that in order to store the same amount of energy with a standard bow, it would be necessary to use a bow with either a greater draw length or a greater maximum draw weight.
One problem with the general design of compound bows, which are currently in use throughout the industry, is the result of unbalanced cable loads. Those unbalanced cable loads are inherent in the basic design generally used in the industry. Specifically, the cable loads are unbalanced because the ends of the cables are not attached in a central position at the ends of the limbs, and during draw these unbalanced cable loads cause a differential bending or twisting of the bow limbs.
A typical compound archery bow comprises a handle with upper and lower limbs extending from the handle. Each limb tip carries a transverse axle upon which an eccentric pulley or cam is rotatably mounted. A bow string and cable arrangement is reeved about the pulleys. There are many different ways in which the bow string can be arranged, but a common arrangement comprises a continuous cable having one end attached to the upper pulley axle, a first cable segment extending across the bow, and wrapping about the lower pulley, a central stretch extending across the bow towards the upper pulley, and a second cable segment wrapping about the upper pulley, and extending across the bow, with the opposite bow string end attaching to the lower pulley axle.
Certain design criteria are normally required for all compound bows. For example, the bow string should be at or near the center of the limb, and if the bow string is off center it must be off center to the left for a right-handed bow. This is because a right-handed bow is generally loaded with an arrow from the left side of the bow. Another design criteria is that the load bearing cables must not interfere with the loading of the arrow and the positioning and flight of the arrow when the bow is shot. These design criteria result in a design for the compound bow where the load bearing cables are generally positioned to the right side of the bow string for a right-handed bow, and with the ends of the cables on the right side of the eccentrically mounted pulley wheel. The positioning of the load bearing cables to the right side provides for the cables to be away from the bow string.
As the bow string is drawn, this provides for variations in load in both the load bearing cables and the drawstring which apply a varying torque to the pulley axle, and these variations in load are generally the reverse of each other. This load effect produces a lateral instability in the bow limbs when the bow string is drawn. The instability is present to a greater or lesser extent and is generally dependent on the relative location of the various load bearing elements along the axles which support the eccentric wheels and the ends of the load bearing cables.
Torque is primarily applied to bow pulleys about axes in two separate planes. The first torque, rotational torque, is applied about the pulley's normal axis of rotation, its rotational mount. The second torque, limb torque, is applied about an axis in a plane normal to the rotational axis of the pulleys. Limb torque attempts to tip the pulleys sideways and is opposed by the limbs, thus developing a torsion in the limb tips.
Rotational torques are applied to the pulley by the force of the bowstring acting in the string groove of the pulley and by the force of the draw cable acting in the cable groove. The lever arm through which each of the draw cable and bow string applies its torque is the distance from the axle to the point of tangency of the cable or string section in the groove. The lever arms are sometimes discussed in terms of the effective diameter of the groove, which is twice the length of the lever arm.
When the pulley is not rotating, such as when the bow is held at full draw, the torques applied by the draw cable and the bow string are necessarily equal. Thus, the force applied by the bow string, F.sub.S, multiplied by the lever arm in the string groove, L.sub.S, equals the force applied by the draw cable, F.sub.C, multiplied by the lever arm in the take-up groove. (F.sub.S .multidot.L.sub.S =F.sub.C .multidot.L.sub.C). The ratio of the lever arm, defined by the bow string in the string groove to the lever arm defined by the draw cable in the cable groove, determines the mechanical advantage supplied by the pulleys to the archer. The greater the mechanical advantage supplied by the pulleys, the higher the force in the draw cable at full draw. (F.sub.C =F.sub.S .multidot.L.sub.S /L.sub.C).
Total limb torque applied to the limbs is determined by the magnitude of the force vectors applied by the string sections and their placement along the pulley axle. The axis about which limb torques are applied, the limb torsional axis, lies normal to the pulley's axis of rotation about the axle and passes somewhere through a central portion of the pulley axle. The exact location of the limb torsional axis depends upon the composition and structure of the limb. Thus, the distance along the axle from the limb torsional axis to the point where a force is applied defines the lever arm for that force, and the limb torque produced by the force equals the magnitude of the force multiplied by the lever arm for the force.
Three sections of the bowstring apply torsion producing forces to the limb tips: the bow string in the string groove, the draw cable in the cable groove, and the fixed end from the opposite pulley draw cable which is typically attached directly to the pulley axle adjacent the cable groove. Typically the bow string is located at one end of the axle, the fixed end at the opposite end of the axle, and the cable groove in between these other two string sections.
There are thus three basic methods to reduce the limb torque: reduce the length of the lever arms through which the forces act, reduce the magnitudes of the forces, or balance the resultant torques. The trend in pulley designs addresses the first method. Today's narrower pulleys and shorter pulley axles minimize the lever arms through which the forces in the cables and string are applied. Reducing the mechanical advantage supplied by the pulleys would also reduce the torque imbalance by reducing the magnitude of the force in the draw cables relative to the force in the bow string, but may have adverse effects upon other aspects of the bow's performance.
In the prior art, there have been attempts to solve this problem of lateral instability through balancing the torques applied to the pulley axle by dividing and redistributing the cable loads along the pulley axle, so as to minimize any lateral instability in the bow limbs. A separate yoke structure for each limb effectively divides each cable load and allows the distribution of that load to the axle on both sides of the pulley. Several methods for dividing the forces in the fixed ends of the cable segments are disclosed in the prior art.
For instance, U.S. Pat. No. 4,202,316 to Barna, issued May 13, 1980, discloses a compound bow having a cable system reeved about pulleys mounted at the distal ends of the bow limbs, and employing somewhat unconventional sprocketed pulleys and a beaded section of the cable engaging the pulleys. The continuous cable arrangement comprises a rigid U-shaped yoke connected to the lower limb tip from whence the cable extends across the bow, around the upper pulley, across the bow, around the lower pulley, across the bow, and terminates in a second U-shaped yoke attached to the tip of the upper limb. Each yoke includes a bifurcated portion, formed by a pair of arm members spaced apart from one another at a first end and connected to one another at a second end. Bores on the first ends of each arm member axially align and mount onto the pulley axle, with the pulley disposed between the arm members. The U-shaped yoke thus distributes the force from the cable equally on both sides of the pulley to reduce limb torque caused by forces in the cables.
U.S. Pat. No. 4,300,521 to Schmitt, issued Nov. 17, 1981, discloses a compound bow wherein the fixed ends of the draw cables are connected to the bow limb tips by pulley-type yoke structures. A cable system is received about draw pulleys located at the distal ends of the bow limbs. Each yoke structure is formed by an idler pulley and a short length of attachment cable wrapped around the idler pulley. The ends of the attachment cable couple to the draw pulley's axle shaft by means of coupling sleeves, mounted one on each side of the pulley. The end of the draw cable loops through an opening in the center of idler pulley to attach the idler pulley to the draw cable. This yoke structure also distributes forces in the draw cable more evenly along the pulley axle to reduce the torque applied to the limb tip when the bow is drawn.
U.S. Pat. No. 5,174,268 to Martin, et al, issued Dec. 29, 1992, discloses a compound archery bow wherein tension adjusting yoke structures attach the fixed ends of the draw cables to the bow limb tips. The yoke assembly appears to comprise a cable tension device having an aperture for receiving an end loop in the draw cable and a second aperture for receiving a yoked loop of attachment cable. The yoked loop of cable extends from an attachment on one portion of an axle mounting the eccentric wheel, through the aperture in the tensioning device, and back towards the axle to be mounted thereto by a second mounting device on an opposite side of the eccentric wheel from the first mounting device. The yoke structure thus distributes the force from the cable equally to reduce limb torque caused by forces in the cables.
Each of the bows disclosed in these three references employs a somewhat complicated yoke structure to distribute the forces from the cables evenly along the pulley axle.
An additional problem in archery bows lies in stringing and unstringing the bow. Particularly in a compound archery bow, when the bow is strung, the cables are under great tension. To unstring the bow, the bow is typically placed into a bow press which presses the ends of the limbs toward one another to relieve the stress on the string. With the tension relieved in the string and cables, the bow may be readily restrung. However, bow presses are typically too bulky to carry into the field, thus making field restringing extremely difficult.